IE, the router goes through POST, then looks at the Flash memory first. If no IOS is found it looks for a TFTP server. If no TFTP server is found it goes to RXBoot, which loads a "cut down" version of the IOS from ROM which has some functionality. If that doesn't work, then you get ROMMON which has even less functionality, but gives you enough to work with to recover the IOS from a back-up somewhere, or by using a nearby router to act as a server so you can copy it's IOS to the dead router. This requires a bit of prep though. Nothing too hard, but a few lines of config need to be gone through and some notes made for things like IP adresses etc.
You can force the router to boot from a specific location by alltering the Configuration Register number at global config.

By far the best resource for these sorts of situations, ie configs that you could do with knowing but don't practice an awful lot, is a book called the
CCNA Portable Command Guide
. I keep one in my laptop bag at all times for those times when I want to TFTP something, or recover an IOS or password recover a switch... etc...